Natural gas is attracting attention as energy source that could substitute for petroleum in future. Since natural gas has combustion characteristics that make itself cleaner than other fossil fuels, it will be highly beneficial to promote the use of natural gas as source of both primary energy and secondary energy from the viewpoint of protection of environment.
From this point of view, massive efforts are currently being paid for the development of technologies for manufacturing methanol, DME (dimethylether) and synthetic oil as well as other substances by way of chemical conversion of natural gas. The so-called indirect conversion method of using synthesis gas that makes a starting material for synthesis is in the mainstream of the technological development. The technologies for manufacturing synthesis gas take an important role in the entire conversion process from the viewpoint of economy.
Known processes for manufacturing synthesis gas include among others (1) the steam reforming process, (2) the ATR (auto thermal reforming) process and (3) the CPOX (catalytic partial oxidation) process.
Since the reaction of the steam reforming process is endothermic, it is necessary to arrange a reaction tube in the furnace of the reforming facility and externally supply heat necessary for reforming. Since heat needs to be supplied at a determined rate, the dimensions of the facility have to be increased proportionally relative to the manufacturing scale. In other words, this process provides little or no scale merit and hence is not suited for large scale manufacturing. While the carbon dioxide reforming that can convert carbon dioxide with steam into synthesis gas is also known, it is accompanied by a similar problem.
The ATR process is a self-heating type reaction process, where oxygen gas is added to the feedstock gas for partial combustion and the heat generated by the combustion is utilized for the following reforming reaction that is an endothermic reaction. In the ATR process, the hydrocarbon in the feedstock gas is partly burned by means of a burner and the generated hot combustion gas (principally containing steam and carbon dioxide that are combustion products as well as unburned feedstock gas) is reformed by a catalyst layer. While this process allows to reduce the dimensions of the facility if compared with the steam reforming process, the facility is still large if it is used for GTL (gas to liquid) production. Therefore, efforts are required to downsize the facility. Additionally, with the ATR process, it is difficult to run the facility in economically optimal conditions because steam needs to be supplied in excess in order to prevent the burner from terminating its service life prematurely and for other reasons.
Finally, the catalytic partial oxidation process is a process of catalytic combustion of a part of material hydrocarbon (which is mainly methane) with a catalyst and reforming the produced hot combustion gas in the same catalyst layer immediately thereafter. While this process is still under investigation and development, it involves only a simple mechanism and is promising in terms of thermal efficiency and productivity. Additionally, it shows a satisfactory reaction performance if the GHSV (gas hourly space velocity) is raised by one digit from the steam reforming process and the ATR process to make it sufficiently adaptable to large scale manufacture. However, the catalytic partial oxidation process is accompanied by a problem that generation of heat is apt to be concentrated near the entrance of the catalyst layer (to produce a so-called hot spot) and therefore sufficient measures need to be taken to prevent the catalyst from being degraded due to high temperatures and the reactor from being damaged.
The generation of a hot spot near the entrance of the catalyst layer in the catalytic partial oxidation process is attributable to that the process for manufacturing synthesis gas involves a two stage reaction system that includes a reaction of partial combustion of methane which is a principal ingredient of the feedstock gas (an exothermic reaction of producing heat at a rate of about 800 kJ/mol) and a subsequent steam reforming reaction of combustion gas (an endothermic reaction of absorbing heat at a rate of about 250 kJ/mol) and a carbon dioxide reforming reaction (an endothermic reaction of absorbing heat at a rate of about 200 kJ/mol) and the heat generation rate at the former stage is very high. If such a reaction system is realized by way of direct catalytic partial oxidation, or a direct reaction system expressed by formula (1) below (an endothermic reaction of producing heat at a rate of about 30 kJ/mol), it will be possible to establish a process that can avoid generation of a hot spot.CH4+1/2O2→CO+2H2  (1)
The above background art is described in International Publications WO97/37929 and WO01/36323 to list a few.